Emulsification of a polymer precursor followed by execution of the polymer chemistry within emulsion droplet reactors provides a facile and versatile method for producing microparticles. Not surprisingly, if a liquid-to-solid chemical reaction proceeds to completion within these drops, the resultant solid particles will possess the shape of the droplets. See, e.g., Nie, Z. H., et al., Polymer particles with various shapes and morphologies produced in continuous microfluidic reactors. Journal of the American Chemical Society, 2005. 127(22): p. 8058-8063 and Dendukuri, D., et al., Controlled synthesis of nonspherical microparticles using microfluidics. Langmuir, 2005. 21(6): p. 2113-2116. Microfluidic flow-focusing devices (MFFDs) provide a straightforward and robust approach to the formation of highly monodisperse emulsion drops. See, e.g., Anna, S. L., N. Bontoux, and H. A. Stone, Formation of dispersions using “flow focusing” in microchannels. Applied Physics Letters, 2003. 82(3): p. 364-366. It has been demonstrated that microfluidic-generated drops can function as both morphological templates and chemical reactors for the synthesis of monodisperse polymer and biopolymer particles. See, e.g., Xu, S., et al., Generation of monodisperse particles by using microfluidics: Control over size, shape, and composition (vol 44, pg 724, 2005); Angewandte Chemie-International Edition, 2005. 44(25): p. 3799-3799; 5. Ikkai, F., et al., New method of producing mono-sized polymer gel particles using microchannel emulsification and UV irradiation. Colloid and Polymer Science, 2005. 283(10): p. 1149-1153; Serra, C., et al., A predictive approach of the influence of the operating parameters on the size of polymer particles synthesized in a simplified microfluidic system. Langmuir, 2007. 23(14): p. 7745-7750; and Zhang, H., et al., Microfluidic production of biopolymer microcapsules with controlled morphology. Journal of the American Chemical Society, 2006. 128(37): p. 12205-12210.
An appealing feature to engineer into emulsion-polymerized particles is porosity. Particles with well-defined pore morphology are essential for many areas of modern technology. Potential applications include catalysis and electrocatalysis, chromatography, and drug delivery. See e. g., Cejka, J. and S. Mintova, Perspectives of micro/mesoporous composites in catalysis. Catalysis Reviews-Science and Engineering, 2007. 49(4): p. 457-509; Hartmann, M., Ordered mesoporous materials for bioadsorption and biocatalysis. Chemistry of Materials, 2005. 17(18): p. 4577-4593; Gasteiger, H. A., et al., Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B-Environmental, 2005. 56(1-2): p. 9-35; Gallis, K. W., et al., The use of mesoporous silica in liquid chromatography. Advanced Materials, 1999. 11(17): p. 1452-1455; and Vallet-Regi, M., et al., A new property of MCM-41: Drug delivery system. Chemistry of Materials, 2001. 13(2): p. 308-311. Precise control over the pore size and shape is crucial for the successful performance of the particles. It allows for optimization of fluid transport in a catalyst, determines the molecular release of solute by a drug delivery vehicle, or defines the size selectivity in chromatography. Templating of oxide materials with surfactant micelles is a powerful method to obtain mesoporous structures with controlled morphology. See e.g., Kresge, C. T., et al., Ordered Mesoporous Molecular-Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature, 1992. 359(6397): p. 710-712. Oxide (e.g., silica) precursor solution (sol) is mixed with a templating surfactant, and evaporation of the solvent leads to an increase in the surfactant concentration. The surfactant forms supramolecular structures according to the solution phase diagram. This is known as evaporative induced self-assembly (EISA) and has been used to obtain bulk porous materials or microparticles using high temperature aerosol methods. See e.g., Lu, Y. F., et al., Aerosol-assisted self-assembly of mesostructured spherical nanoparticles. Nature, 1999. 398(6724): p. 223-226; Brinker, C. J., et al., Evaporation-induced self-assembly: Nanostructures made easy. Advanced Materials, 1999. 11(7): p. 579-585; and Bore, M. T., et al., Hexagonal mesostructure in powders produced by evaporation-induced self-assembly of aerosols from aqueous tetraethoxysilane solutions. Langmuir, 2003. 19(2): p. 256-264; Alternatively, mesoporous particle synthesis via EISA can be performed in water in oil emulsion droplets under milder temperature stresses. Recently, Andersson et al. demonstrated the synthesis of spherical mesoporous silica particles using an approach that combines previously established emulsion-based precipitation methods with the EISA method. See, e.g., Andersson, N., et al., Combined emulsion and solvent evaporation (ESE) synthesis route to well-ordered mesoporous materials. Langmuir, 2007. 23(3): p. 1459-1464; Schacht, S., et al., Oil-water interface templating of mesoporous macroscale structures. Science, 1996. 273(5276): p. 768-771; and Huo, Q. S., et al., Preparation of hard mesoporous silica spheres. Chemistry of Materials, 1997. 9(1): p. 14-17. This synthesis route, referred to as the emulsion and solvent evaporation method (ESE), produced well-ordered 2D hexagonal mesoporous silica microspheres. The advantages of this method are control of synthesis parameters such as emulsion droplet size, temperature, evaporation speeds, humidity, and the composition of the surfactant solution. In comparison to aerosol methods, the relatively slower evaporation rate of the solvent from the emulsion droplets allows a high-degree of homogeneity of the components in the liquid crystalline phase prior to fossilization of the structures by silica condensation. This is perhaps the most important distinction of the emulsion EISA method from aerosol-based EISA methods.
Surfactant self-assembly provides a powerful method for synthesizing mesoporous materials. However, these materials are limited to micelle-templated pores with diameters of a few nanometers, and mesoporous microparticles synthesized by aerosol and ESE methods enclose internal structures rendered inaccessible at the surface due to inherent formation of a solid material layer at the surface. See, e.g., Bore, M. T., et al., and Andersson, N., et al., cited above. To address the requirements of emerging technologies, the next generation of porous oxide materials must be highly structured and functionalized. Hierarchically porous structures offer advantages in design of materials where catalytic activity is to be utilized in immediate conjunction with transport of reactants. Templating approaches for hierarchical material fabrication are attractive, as they can be combined with other methods such as impregnation or precipitation to yield structures with controlled porosity, surface chemistry, and hydrophilicity or hydrophobicity. Interfacial phenomena such as spontaneous formation of complex microemulsion phases present exceptional and generally less-explored avenues for particle nanostructure templating. See e.g., Hu, Y. and J. M. Prausnitz, Molecular Thermodynamics of Partially-Ordered Fluids—Microemulsions. Aiche Journal, 1988. 34(5): p. 814-824 and Nagarajan, R. and E. Ruckenstein, Molecular theory of microemulsions. Langmuir, 2000. 16(16): p. 6400-6415.
Furthermore, developing pathways for fabrication of monodisperse oxide particles with hierarchical internal nanoporous structure provides additional opportunities and level of complexity of material structural design. Same-size spherical microparticles are essential for developing novel families of functional digital inks for printing catalysts, current collectors, and creating polymer/oxide composites at the microscale. Such inks are essential for miniaturization of devices for catalysis, sensing and detection, and microfuel cells. Manipulating the particle structure, pore morphology and surface chemistry allows for better control at the micro and nanoscale. Monodisperse spherical microparticles can be ordered in 2D and 3D arrays to create structures with hierarchical porosity. Such structures will exhibit a variety of characteristic pore dimensions: (i) nanoscale pores that are due to micelle (single nanometer) and microemulsion (tens of nanometers) templating and (ii) microscale pores determined by the voids between monodispers microparticles in the array.